There can also be software faults. I've recently been suffering from some very poor battery life. The reason is that the Kindle lets you search across all installed books. To support this it constructs an index of all the words in each book. Building the index takes a lot of electrical power. Normally it happens once when a new book is added. Sometimes the indexing process gets stuck, never succeeds in indexing the book, and runs forever. Then battery life drops from 2 months to a week or so. A fix is to delete the book, then reboot the Kindle.

Presumably it only happens if there is some kind of formatting issue with the offending book. I've only seen it with books that weren't bought from Amazon. (I've been buying eBooks for a decade or more, and have many that predate the Kindle.)

The point is, it can go wrong, and even if its working now, Amazon can update the software at any time, and the new software could have a new bug. Their system is not something you should trust your life to. It's not designed for that.